The present invention relates to poreless, non-sintered plate-like magnetite and plate-like maghemite particles having an average diameter of more than 0.5 .mu.m and not more than 2.0 .mu.m, and a specific surface area of not more than 6.0 m.sup.2 /g, and processes of producing the same. The plate-like magnetite particles and plate-like maghemite particles of the present invention are useful to materials for manufacturing electromagnetic wave absorbers and shields.
The use of magnetite particles and maghemite particles as the manufacturing materials for the electromagnetic wave absorbers and shields is known.
This fact is disclosed, for example, Japanese Patent Application Laid-Open (KOKAI) No. 54-110496 (1979) discloses: "the electromagnetic wave absorption properties of magnetite (Fe.sub.3 O.sub.4) were examined in comparison to those of conventional composite ferrites and in the microwave region of 5-10 GHz, the material can be made thinner, and has better workability and superior electromagnetic wave absorption to composite ferrites."
Electromagnetic wave absorption and shielding are carried out by applying a paint obtained by preparing a dispersion of magnetite particles or maghemite particles in a vehicle, to buildings, ships and aircraft which reflect such waves, or to instruments which generate such waves.
It is required that the magnetite or maghemite particles used as materials for electromagnetic wave absorption and shield materials are plate-like particles, have excellent dispersibility in the vehicle, and have a large particle size, specifically a particle size of not less than 0.5 .mu.m.
Concerning the shape of the particles, it is disclosed, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 61-77626 (1986) that: "plate-like ferrite particles have excellent properties as electromagnetic wave absorption and shielding materials in the low frequency region, and these properties are not achieved by ordinary ferrite particles"; in Japanese Patent Application Laid-Open (KOKAI) No. 55-104923 (1980) that: "a very remarkable parallel orientation of individual particles is produced in the covering material. Accordingly, a very high packing density can thus be achieved, and as a result, for example, anti-corrosion is enhanced, screening of electromagnetic interference becomes more effective, and electrical conductivity is increased"; and "there are other uses for hexagonal thin-plate (plate-like) iron oxide particles having a magnetite or maghemite structure . . . a very remarkable parallel orientation of individual particles is produced. A very high packing density is therefore, can be achieved." It is, therefore, evident that plate-like particles are necessary.
Next, with regard to the particle size, it is disclosed in the above Japanese Patent Application Laid-Open (KOKAI) No. 61-77626 (1986) that: "the average major diameter of the particles is not less than 0.5 .mu.m".
Conventionally, a process for producing plate-like magnetite particles is known, for example:
(i) a process comprising steps of autoclaving an alkaline suspension containing ferric hydroxide or goethite to form plate-like hematite particles in aqueous solution, and reducing the thus obtained plate-like hematite particles under heating in a reducing gas; and PA0 (ii) a process comprising steps of rapidly oxidizing an alkaline suspension containing ferrous hydroxide with a strong oxidizing agent, or reacting a ferric salt with alkali in an aqueous medium in the presence of specific additives to produce ferric hydroxide, autoclaving the thus obtained ferric hydroxide, thereby obtaining plate-like goethite particles in aqueous solution dehydrating the said plate-like goethite particles under heating, and reducing the obtained particles under heating in a reducing gas. PA0 (iii) a process comprising steps of autoclaving an alkaline suspension containing ferric hydroxide or goethite to form plate-like hematite particles in aqueous solution, reducing the thus obtained plate-like hematite particles under heating in a reducing gas, and then oxidizing the obtained particle under heating; and PA0 (iv) a process comprising steps of rapidly oxidizing an alkaline suspension containing ferrous hydroxide with a strong oxidizing agent, or reacting a ferric salt with alkali in an aqueous medium in the presence of specific additives to produce ferric hydroxide, autoclaving the thus obtained ferric hydroxide, thereby obtaining plate-like goethite particles in aqueous solution, dehydrating the said plate-like goethite particles by heating, reducing the obtained particle under heating in a reducing gas, and then oxidizing the obtained particles under heating.
Also, a process for producing plate-like maghemite particles is known, for example:
The processes (i) and (iii) are disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 51-28700 (1976) and Japanese Patent Application Laid-Open (KOKAI) No. 55-104923 (1975). The processes (ii) and (iv) are disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 61-266311 (1986) and Japanese Patent Application Laid-Open (KOKAI) No. 55-104923 (1980).
Plate-like magnetite and maghemite particles having excellent dispersibility and large size, and in particular, having a particle size of more than 0.5 .mu.m are, therefore, strongly demanded at the present. However, in the case of the known processes mentioned above, it is necessary to reduce the plate-like particles formed in aqueous solution under heating in a reducing gas. In this reducing process, rapid particle growth takes place, whereby the sintering of particle and between particles arises. As a result, it becomes difficult to disperse the particles in the vehicle or the resin, and accordingly the packing density lowers and particles orientation is impaired.
It is well-known that when plate-like magnetite particles are oxidized under heating so as to form plate-like maghemite particles, particle growth does not take place and, therefore, the sintering of particle and between particles do not occur.
Further, in the case of the processes (ii) and (iv), water in goethite crystals is dehydrated when plate-like goethite particles are heated, and many pores are, therefore, produced on the surfaces of and inside the obtained plate-like magnetite particles. When the plate-like maghemite particles are obtained by oxidizing the plate-like magnetite particles under heating, many pores remain on the surfaces of and inside the particles. If such porous plate-like magnetite or plate-like maghemite particles are dispersed in a vehicle or a resin, other fine particles are attracted to the portions where there are superficial magnetic poles. As a result, many particles agglomerate together, and agglomerated lumps of fairly large size are formed. The dispersion of the particles in a vehicle is then rendered difficult, and the particle orientation is impaired.
As is clear from the above discussion, in order to obtain poreless, non-sintered maghemite particles, plate-like magnetite particles should be formed directly in aqueous solution.
The present inventors, in order to overcome the above disadvantages, already proposed a process of forming directly poreless, non-sintered plate-like magnetite particles in aqueous solution (European Patent No. 0279626A). Using this process, however, plate-like magnetite particles of a large particle size, specifically of a particle size of more than 0.5 .mu.m, could not be obtained.
The coercive force and residual magnetization of magnetite particles is correlated with particle size. As the size increases, the coercive force and residual magnetization lower, so particles of large size are preferable for electromagnetic wave absorbers and shielding materials.
It is, therefore, strongly demanded to offer a process for directly forming large particles of plate-like magnetite, and especially plate-like magnetite particles having a particle size of more than 0.5 .mu.m in aqueous solution.
As a result of the present inventor's studies for forming plate-like magnetite particles directly in aqueous solution, and forming plate-like maghemite particles of large size, especially plate-like maghemite particles having a particle size of larger than 0.5 .mu.m, it has been found that by autoclaving an alkaline suspension of ferric oxide hydroxide particles and ferrous hydroxide in the presence of sulfate ion in the temperature of 200.degree.-300.degree. C., and as occasion demands, oxidizing the thus obtained plate-like magnetite particles obtained under heating in air, the obtained plate-like magnetite and plate-like maghemite particles are poreless, non-sintered particles having an average diameter of more than 0.5 .mu.m and not more than 2.0 .mu.m and a specific surface area of not more than 6.0 m.sup.2 /g. Based on this finding, the present invention has been attained.